WATER RESOURCES IN ARID REGIONS 107 
Finally there is another source of supply: overflows coming from 
nearby aquifers. Thus in Tunisia there is a series of aquifers in 
miocene sand formations whose water naturally springs along 
watertight faults. The waters are partially used for irrigation. 
The waters not used, joined with waters strained out and perco- 
lated from the irrigations, reinfiltrate and contribute to the sup- 
ply of a compartment downstream. Water is in this way trans- 
ferred from compartment to compartment, but each passage to 
the surface of the soil, or near it, corresponds to an increase in 
salinity. Sometimes, therefore, it is better to use as well as possible 
the waters in the upstream compartments, without troubling 
much about the decrease in supply of downstream compartments. 
Hydrological considerations of the salinity of the water have an 
importance as great as the economic findings for the planning 
of systems of irrigation. Suppose a cycle of events wherein the 
water percolated from upstream irrigations is reused for other 
irrigation. On hydrological grounds the full utilization of under- 
ground water requires the construction of watertight irrigation 
systems as soon as the excess waters come to percolate into water 
strata which have too much saline content to be recoverable for 
other irrigation. This is the case for the artesian aquifers of the 
Tunisian south. As long as downstream water strata which receive 
percolation from upstream irrigation can be reused without too 
much salinity, the irrigation ditches need not be lined unless 
otherwise warranted. 
The recharge of an aquifer by another can also take place by 
underground communication. If the aquifers are of small extent, 
in regions not very arid, they can be studied by comparing the 
chemical composition of the waters and by examining the hydro- 
logical data (11). In the case of very extensive units such as the 
miocene and cretacean basins in the south of the Saharian Atlas, 
the problem becomes much more difficult and has not yet been 
solved for lack of sufficient information concerning the geology, 
the deep hydrology, and the topography of the land (3, 12). 
Discharge. Apart from the underground water which returns 
to the surface in a form rather easily measurable—springs and 
wells—some is lost to the sea by underwater seepage or evaporates 
